Improved composition and method for removal of carbonylsulfide from acid gas containing same

ABSTRACT

Novel solvent composition for selective removal of COS from a gas stream containing same, said composition comprising a) at least one polyalkylene glycol alkyl ether of the formula (I) or 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone or a mixture of N-formylmorpholine and N-acetylmorpholine Wherein R 1  is an alkyl group having from 1 to 6 carbon atoms; R 2  is hydrogen or an alkyl group having from 1 to 4 carbon atoms; Alk is an alkylene group, branced or unbranced, having from 2 to 4 carbon atoms, and n is from 1 to 10; and b) at least one alkanolamine compound of the formula (II) or at least one piperazine compound of formula (III) wherein R 3  is hydrogen, an alkyl group having from 1 to 6 carbon atoms, or the R 4 OH group; R 4  is a branched or unbranched alkylene group having from 1 to 6 carbon atoms; R 5 , independently in each occurrence, is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; and R 6  is hydrogen, an alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl group having from 1 to 4 carbon atoms. 
 
R 1 O-(Alk-O) n —R 2   (I) 
 
R 3 NHR 4 OR 6   (II)

The present invention relates to an improved composition for selectiveremoval of carbonyl sulfide (COS) with minimal absorption of carbondioxide (CO₂) from an acid gas containing COS and CO₂, and also to amethod for selective removal of COS from an acid gas containing COS andCO₂ using this improved composition.

Natural and synthesis gas streams, derived from natural gas reservoirs,petroleum or coal, often contain a significant amount of carbonylsulfide (COS) in addition to other impurities such as carbon dioxide(CO₂), hydrogen sulfide (H₂S), sulfur dioxide (SO₂), carbon disulfide(CS₂), and mercaptans. Various compositions and processes for removal ofacid gases, including COS, from a gas containing same are known anddescribed in the literature.

Certain physical solvents have been widely used for the selectiveremoval of H₂S from gas streams containing H₂S, CO₂ and optionally,other components. For example, dialkyl ethers of polyalkylene glycolsare commonly specified for this purpose. Typical of the numerousdisclosures of such solvents in the art is European Patent ApplicationNo. EP 0770420 A2 and U.S. Pat. Nos. 3,737,392; 3,824,766; 3,837,143;4,044,100; 4,336,233; 4,581,154; 4,741,745; and 4,946,620, among others.According to such references, it is well known that the solubility ofH₂S in these glycols is much higher than the solubility of CO₂. Thisdifference in solubility forms the basis for the selective removal ofH₂S by glycol ethers and other physical solvents. Unfortunately, thesolubility of COS in most physical solvents is not very different fromthe solubility of CO₂. Hence, selective removal of COS or COS and H₂Sfrom gas streams containing CO₂ and optionally other components, hasproved difficult.

Certain tertiary alkanolamines (methyldiethanolamine and triethanolaminefor example) have been widely used for the selective removal of H₂S fromgas streams which also contain CO₂. Users of these amines exploit therapid reaction with H₂S and the slow reaction with CO₂ to allowselective removal of H₂S. Unfortunately, the reaction of these amineswith COS is about 100 times slower than the reaction with CO₂. Hence,tertiary amines are relatively ineffective at separating COS or COS andH₂S from CO₂ containing streams.

Certain primary and secondary amines (monoethanolamine, diethanolamine,methylethanolamine, aminoethoxy ethanol, piperazine for example) havebeen widely used for the simultaneous removal of H₂S and CO₂. Theseamines react rapidly with both H₂S and CO₂ and are well suited forsimultaneous removal of H₂S and CO₂. Although the reaction of COS withthese amines is also about 100 times slower than the reaction with CO₂,appreciable amounts of CO₂ and COS can be removed. Unfortunately, sinceCOS reacts much more slowly with these amines than does CO₂, selectiveremoval of COS or COS and H₂S with primary or secondary amines hasproved difficult.

The selective removal of COS or COS and H₂S from gas streams containingCO₂ and optionally other components has proved difficult with bothphysical and chemical solvents. However, in a number of conventionalhydrocarbon processing techniques, it is desirable or necessary toremove COS down to few parts per million (ppmv) for several reasons,such as catalyst sensitivity to COS in subsequent operations andstatutory or contract requirements regarding sulfur content in productor waste gas streams. The presence of COS has also been identified asthe cause of degradation reactions in several commercial processes foracid gas removal. However, in many cases it is neither necessary nordesirable to remove CO₂ from gaseous mixtures.

Various solvent compositions and methods are known for selective removalof COS and H₂S from gaseous mixtures containing same and other acidicgases such as CO₂.

U.S. Pat. No. 3,989,811 (Hill) discloses a multi-step process forremoval of acid gases, that is, H₂S, CO₂, and sulfur containingcompounds including CS₂, COS, and various mercaptans from sour gases. Inthis multi-step process, H₂S, CO₂ and COS are absorbed in a nonselectivealkanolamine. The regenerated acid gases are then contacted with aselective amine to produce an H₂S rich stripper off gas and a lowpressure CO₂ rich stream. Suitable alkanolamines includemethyldiethanolamine, triethanolamine, or one or more dipropanolamines,such as di-n-propanolamine or diisopropanolamine. The preferredabsorbent for treatment of COS containing gas streams also contains asignificant amount of a tetramethylene sulfone (the unsubstitutedcompound is known as sulfolane). In principle, selective removal of COSand H₂S from the original sour gas stream could be achieved if the CO₂rich product stream were recompressed and combined with the main treatedgas stream. However, the capital and operating costs associated withrequiring two absorbers, two strippers and compression of the CO₂ streamwould weigh against this option, assuming a less costly alternative canbe found.

U.S. Pat. No. 4,482,529 (Chen et al.) discloses a single step processfor the selective removal of COS from a gas stream containing CO₂. Smallamounts of bicyclo tertiary amine are added to a physical solventalready known to be selective for H₂S in the presence of CO₂. Accordingto the invention, addition of the bicyclo tertiary amine encourageshydrolysis of the COS to H₂S and CO₂. The net effect is to improve theabsorption of COS without substantially increasing the absorption ofCO₂. Applicable physical solvents include SELEXOL™ solvent (a blend ofpolyethylene glycol dimethyl ethers sold by Union Carbide Corporationunder the trademark SELEXOL), sulfolane, methanol and others.

U.S. Pat. No. 4,524,050 (Chen et al.) discloses a process forhydrolyzing COS in gas streams to H₂S and CO₂ using solid supportedbicyclo tertiary amine catalyst.

U.S. Pat. No. 4,504,449 (Doerges et al.) discloses a process forselective removal of H₂S and possibly COS from a CO₂ containing gasstream using highly volatile secondary amines in an organic solvent. Acomplex regeneration scheme is required. Applicable secondary amines arediethyl amine, diisopropyl amine, methyl isopropyl amine, ethylisopropyl amine, dipropyl amine, methyl n-butylamine, methyl isobutylamine and methyl sec-butyl amine. Applicable organic solvents includealkylated polyethylene glycol ethers, tetrahydrothiophene dioxide etc.

U.S. Pat. No. 4,532,116 (Doerges et al.) discloses a process for theremoval of H₂S and/or COS from synthesis gas using at least onesecondary amine and an organic solvent. Although the process isselective for H₂S and COS in the presence of CO₂, the process iscomplex. Two scrubbing zones are required—a preliminary desulfurizingzone using a conventional circulated and regenerated solution and a finedesulfurizing or scrubbing zone using unregenerated solvent. The finedesulfurizing zone contains a heat exchanger to condense theunregenerated solvent. Applicable organic solvents are methanol, ethanoland isopropanol. Applicable secondary amines are N-ethylethaneamine,N-(1-methylethyl)propaneamine, N-methyl-2-propanamine,N-ethyl-2-propanamine, N-propyl-1-propanamine, N-methyl-1-butanamine,N-2-dimethyl-1-propanamine, N-methyl-2-butanamine.

U.S. Pat. No. 4,749,555 (Bush) discloses a process for selective removalof H₂S and COS without absorbing CO₂ from a gas stream having arelatively large concentration of CO₂ using bridgehead amines(bicyclotertiary amine or bicyclo amidine), tertiary amine, physicalsolvent and water. Applicable physical solvents include sulfolane,polyethylene glycol dimethyl ethers and others.

U.S. Pat. No. 4,980,140 (Souby) discloses a process for selectiveremoval of COS from H₂S using a solvent comprising a tertiary amine, aphysical co-solvent and a minor amount of water. The useful physicalco-solvent includes sulfones, sulfoxides, glycols and the mono anddiethers thereof, 1,3-dioxo heterocyclic compounds (dioxane anddioxolane), aromatic ethers, aromatic hydrocarbons and N-alkylatedlactams of gamma or omega amino acids.

U.S. Pat. No. 5,413,627 (Landeck et al.) discloses the selective removalof H₂S and COS from CO₂ using a physical scrubbing agent comprising aheterocycle having five or more ring atoms, which contains twoheteroatoms, one of which is nitrogen and the other of which is oxygenor nitrogen. The nitrogen atom present in the ring is/are either doublebonded or single bonded but, if single bonded, the nitrogen isorgano-substituted. A wide variety of scrubbing agents are disclosed,including 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMTP).

U.S. Pat. No. 5,589,149 (Garland et al.) discloses absorption solventsfor removing mercaptans from gas streams without the use of iodine. Thesolvent comprises alkyl ether of a polyalkylene glycol and a secondarymonoalkanolamine and, optionally, other amines, such as dialkanolamines.

U.S. Pat. No. 6,277,345 B1 (Stankowiak et al.) discloses the use ofabsorption liquid comprising at least one dialkanolamine, at least onepolyalkylene glycol alkyl ether, and water for nonselective removal ofCO₂, H₂S, COS and other acidic gases from a gaseous stream.

U.S. Pat. No. 6,102,987 (Gross et al.) discloses a process for removalof CO₂ and sulfur compounds from natural gas and raw synthesis gas witha mixture of N-formylmorpholine and N-acetylmorpholine at temperaturesbetween −20° C. and +40° C. at pressure of 10 to 150 bar in absorbingoperation.

There is still a need in the industry for an improved composition forselective removal of COS from gaseous streams containing COS and CO₂. Ithas now been surprisingly and unforeseeably found out that the additionof a primary alkanolamine, secondary alkanolamine or piperazine compoundto a polyalkylene glycol alkyl ether or1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone (DMPT) results inselective removal of COS from the gaseous streams containing same withminimal removal of CO₂.

One aspect of the present invention concerns a solvent composition forremoval of COS from a gas stream containing same, said compositioncomprising

-   -   a) at least one polyalkylene glycol alkyl ether of the formula        R₁O-(Alk-O)_(n)—R₂  (I)    -   wherein R₁ is an alkyl group having from 1 to 6 carbon atoms; R₂        is hydrogen or an alkyl group having from 1 to 4 carbon atoms;        Alk is an alkylene group, branched or unbranched, having from 2        to 4 carbon atoms, and n is from 1 to 10; and    -   b) at least one alkanolamine compound of the formula        R₃NHR₄OR₆  (II)        -   or        -   at least one piperazine compound of formula    -   wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon        atoms, or the R₄OH group; R₄ is a branched or unbranched        alkylene group having from 1 to 6 carbon atoms; R₅,        independently in each occurrence, is hydrogen or an hydroxyalkyl        group having from 1 to 4 carbon atoms; and R₆ is hydrogen, an        alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl        group having from 1 to 4 carbon atoms.

Another aspect the present invention concerns a solvent composition forremoval of COS from a gas stream containing same, said compositioncomprising

-   -   a) 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; and    -   b) at least one alkanolamine compound of the formula        R₃NHR₄OR₆  (II)        -   or        -   at least one piperazine compound of formula    -   wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon        atoms, or the R₄OH group; R₄ is a branched or unbranched        alkylene group having from 1 to 6 carbon atoms; R₅,        independently in each occurrence, is hydrogen or an hydroxyalkyl        group having from 1 to 4 carbon atoms; and R₆ is hydrogen, an        alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl        group having from 1 to 4 carbon atoms.

In another aspect, the present invention concerns a process forselective removal of COS from a gas stream containing same, said processcomprising treating the gas stream with a solvent composition comprising

-   -   a) at least one polyalkylene glycol alkyl ether of the formula        R₁O-(Alk-O)_(n)——R₂  (I)    -   wherein R₁ is an alkyl group having from 1 to 6 carbon atoms; R₂        is hydrogen or an alkyl group having from 1 to 4 carbon atoms;        Alk is an alkylene group, branched or unbranched, having from 2        to 4 carbon atoms; and n is from 1 to 10; and    -   b) at least one alkanolamine compound of the formula        R₃NHR₄OR₆  (II)        -   or        -   at least one piperazine compound of formula    -   wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon        atoms, or the R₄OH group; R₄ is a branched or unbranched        alkylene group having from 1 to 6 carbon atoms; R₅,        independently in each occurrence, is hydrogen or an hydroxyalkyl        group having from 1 to 4 carbon atoms; and R₆ is hydrogen, an        alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl        group having from 1 to 4 carbon atoms.

Still in another aspect, the present invention concerns a process forselective removal of COS from a gas stream containing same, said processcomprising treating the gas stream with a solvent composition comprising

-   -   a) 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; and    -   b) at least one alkanolamine compound of the formula        R₃NHR₄OR₆  (II)        -   or        -   at least one piperazine compound of formula    -   wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon        atoms, or the R₄OH group; R₄ is a branched or unbranched        alkylene group having from 1 to 6 carbon atoms; R₅,        independently in each occurrence, is hydrogen or an hydroxyalkyl        group having from 1 to 4 carbon atoms; and R₆ is hydrogen, an        alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl        group having from 1 to 4 carbon atoms.

Still in another aspect, the present invention concerns a solventcomposition for removal of COS from a gas stream containing same, saidcomposition comprising

-   -   a) a mixture of N-formylmorpholine and N-acetylmorpholine; and    -   b) at least one alkanolamine compound of the formula        R₃NHR₄OR₆  (I)        -   or        -   at least one piperazine compound of formula    -   wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon        atoms, or the R₄OH group; R₄ is a branched or unbranched        alkylene group having from 1 to 6 carbon atoms; R₅,        independently in each occurrence, is hydrogen or an hydroxyalkyl        group having from 1 to 4 carbon atoms; and R₆ is hydrogen, an        alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl        group having from 1 to 4 carbon atoms.

Still in another aspect, the present invention concerns a process forselective removal of COS from a gas stream containing same, said processcomprising treating the gas stream with a solvent composition comprising

-   -   a) a mixture of N-formylmorpholine and N-acetylmorpholine; and    -   b) at least one alkanolamine compound of the formula        R₃NHR₄OR₆  (II)        -   or        -   at least one piperazine compound of formula    -   wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon        atoms, or the R₄OH group; R₄ is a branched or unbranched        alkylene group having from 1 to 6 carbon atoms; R₅,        independently in each occurrence, is hydrogen or an hydroxyalkyl        group having from 1 to 4 carbon atoms; and R₆ is hydrogen, an        alkyl group having from 1 to 6 carbon atoms or an hydroxyalkyl        group having from 1 to 4 carbon atoms.

As used herein, the terms “gas”, “acid gas”, and “gaseous stream” areintended to refer to natural gas, hydrocarbon gas, synthesis gas, steamreformer-type gases, and any other gas containing COS, CO₂ and othergaseous components such as hydrogen sulfide, methane, ethane, propane,hydrogen, carbon monoxide, mercaptans etc. The expression “minimaladditional removal of CO₂” means additional CO₂ absorption due to thepresence of the amine additive is less than one third of the CO₂ presentin the feed gas, all other factors being the same.

In accordance with the present invention, it has been surprisinglydiscovered that the solvent compositions of the present invention haveexcellent selectivity for the removal of COS from a gaseous streamcontaining same with minimal additional removal of CO₂.

It was completely surprising and unexpected to find out addition of analkanolamine compound of formula II above or a piperazine compound offormula III above to least one polyalkylene glycol alkyl ether of theformula I above, and in particular to a mixture of polyalkylene glycolethers of formula I above, such as the SELEXOL™ solvent, removessubstantially more of the COS present in the gaseous streams withminimal additional removal of CO₂.

It was also equally surprising and unexpected to find out that theaddition of an alkanolamine compound of formula II above or a piperazinecompound of formula III above to1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone removes substantiallymore of COS present in the gaseous streams with minimal additionalremoval of CO₂

Polyalkylene alkyl ethers of formula I suitable in the practice of thepresent invention are well known and include, without any limitation,diethylene glycol dimethyl ether, diethylene glycol diisopropyl ether,triethylene glycol dimethyl ether, triethylene glycol diisopropyl ether,tetraethylene glycol dimethyl ether, tetraethylene glycol diisopropylether, pentaethylene glycol dimethyl ether, pentaethylene glycoldiisopropyl ether, hexaethylene glycol dimethyl ether, hexaethyleneglycol diisopropyl ether, heptaethylene glycol dimethyl ether,octaethylene glycol dimethyl ether, nonaethylene glycol dimethyl ether,decaethylene glycol dimethyl ether, and any mixture thereof.

Preferred polyethylene glycol alkyl ether is a mixture consisting of adimethyl ethers of polyethylene glycols of formula CH₃O(C₂H₄O)_(n)CH₃wherein n is from 2 to 10. Particularly preferred polyethylene glycolalkyl ether is the mixture of dimethyl ethers of polyethylene glycolssold under the trademark SELEXOL by Union Carbide Corporation. SELEXOL™solvent is a mixture of dimethyl ethers of polyethylene glycolscomprising from 0 to 0.5 wt percent of diethylene glycol dimethyl ether,from 5 to 7 wt percent of triethylene glycol dimethyl ether, from 16 to18 wt percent tetraethylene glycol dimethyl ether, from 23 to 25 wtpercent of pentethylene glycol dimethyl ether, from 22 to 24 wt percentof hexaethylene glycol dimethyl ether, from 15 to 17 wt percent ofheptaethylene glycol dimethyl ether, from 8 to 10 wt percent ofoctaethylene glycol dimethyl ether, from 3 to 5 wt percent ofnonaethylene glycol dimethyl ether, and from 1 to 2 wt percent ofdecaethylene glycol dimethyl ether. SELEXOL™ solvent is widely used ingas treating applications for the removal of acidic gases. However, itis known that the removal of COS by the SELEXOL™ solvent is poor due tosolvent factors (low solubility) and applications specific factors (lowCOS partial pressure). Certain amine additives such as bicyclic tertiaryamines and tertiary amines have been used, with some success, to improveremoval of COS by SELEXOL™ solvent. It has now been discovered that theSELEXOL™ solvent containing alkanolamines of formula II hereinbefore ora piperazine compound of formula m hereinabove is surprisingly selectivein removing COS in the presence of CO₂ from gases.

Alkanolamine compounds of formula II useful in the practice of thepresent invention are well known compounds and include both primary andsecondary alkanolamines. Preferred alkanolamine compounds of formula IIare primary alkanolamines. Non-limiting examples of alkanolaminecompounds are monoethanolamine (MEA), diethanolamine (DEA),methylethanolamine (NMEA), diisopropanolamine (DIPA), and2-(2-aminoethoxy)ethanol (AEE) also known as diethylene glycolamine(available from Huntsman Corporation under the trademark DGA).

The present invention is particularly useful in the desulfurizationportion of an Integrated Combined Cycle Gasification (IGCC) processusing an IGCC system consisting of a conventional SELEXOL™ solvent acidgas removal unit, a fixed bed catalytic converter for reducing the gasphase concentration of carbonyl sulfide (COS), and heat exchangerequipment upstream of the SELEXOL™ solvent unit. It has beensurprisingly found out that the addition of the alkanolamine compound offormula II above to the SELEXOL™ solvent results in selective removal ofCOS from the gas feed in the presence of CO₂ with minimal removal ofCO₂. The costs associated with the fixed bed catalytic converter arealso significantly reduced.

Since the process of the present invention for selective removal of COSfrom gaseous streams is conducted in the desulfurization portion of anIntegrated Combined Cycle Gasification (IGCC) process, the operatingconditions of the process are those of the IGCC process. These operatingconditions are well known to a person of an ordinary skill in the art.

All parts, percentages and ratios herein are by weight unless otherwiseindicated.

The invention will be further clarified by a consideration of thefollowing examples that are intended to be purely exemplary of thepresent invention and not limiting in any way.

The following components are employed in the examples.

MEA is monoethanolamine;

DEA is diethanolamine;

TEA is triethanolamine;

NMEA is methylethanoalmine;

DIPA is diisopropanolamine;

HEP is hydroxyethylpiperazine;

MDEA is methyldiethanolamine;

DMEA is dimethyethanolamine;

DBU is 1,8-diazabicyclo[5.4.0]undec-7-ene;

DBN is 1,5-diazabicyclo[4.3.0]non-5-ene;

DABCO is 1,4-diazabicyclo[2.2.2]octane, sold by Air Products andChemicals, Inc. under the trademark DABCO;

DMTP is 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone;

Quinuclidine is 1,4-ethanolpiperidine; and

AEE is 2-(2-aminoethoxy)ethanol.

EXAMPLES 1 TO 10 AND COMPARATIVE EXAMPLES 1 TO 12

The performance of SELEXOL™ solvent alone, and the mixture of SELEXOL™solvent and prior art additives were evaluated in a bench scale glassabsorber-stripper apparatus with the following characteristics:

-   -   a) standard gas feed conditions (10 mole percent CO2, 1.6 mole        percent COS, and the balance nitrogen, about 3 L/min, 80° F.),    -   b) standard liquid feed conditions (about 11 cc/min, 80° F.),    -   c) standard amount of absorber and stripper staging (small        trays, 25 absorber, 20 stripper),    -   d) approximately standard amount of stripping heat duty applied,    -   e) standard amount of the additive used about 3 percent by        weight),    -   f) gas phase analysis by gas chromatography, liquid phase water        content by Karl Fisher method,    -   g) careful attention to ensuring steady state operation,    -   h) liquid phase COS content was not measured, and    -   i) stripper overhead not sampled for COS or H₂S.

The results obtained in the performance evaluation of different gastreating solvent are provided in Tables below. TABLE 1 Evaluation ofPrior Art and Various Additives for Removal of COS by SELEXOL ™ solventCOS CO₂ Absorption Absorption (percent of (percent of Solvent feed)feed) SELEXOL ™ solvent* 0.7 0.7 SELEXOL ™ solvent* 1.2 0.2 S + 3 wtpercent 1,4 2.6 0.0 diazabicyclo[2.2.2]octane (DABCO)* S + 3 wt percentquinuclidine* 5.1 −0.2 S + 3 wt percent 3.1 −0.61,5-diazabicyclo[4.3.0]non-5- ene (DBN)* S + 3 wt percent 2.5 0.51,8-diazabicyclo[5.4.0]undec- 7-ene (DBU)* S + 3 wt percenttriethanolamine 2.0 0.5 (TEA)* S + 3 wt percent triethanolamine −0.7 2.7(TEA)* S + 3 wt percent methyldiethanol- 1.3 1.3 amine (MDEA)* S + 3 wtpercent dimethylethanol- 2.0 0.0 amine (MDEA)* S + 3 wt percenthydroxyethylpi- 17.1 0.7 perazine (HEP) S + 3 wt percent diisopropanol-18.9 1.0 amine (DIPA) S + 3 wt percent diethanolamine 18.9 2.8 (DEA) S +3 wt percent diethanolamine 28.3 0.1 (DEA) S + 3 wt percent monoethanol-28.3 −0.7 amine (MEA) S + 3 wt percent piperazine 29.1 0.8 S + 3 wtpercent methylethanol- 6.6 −0.6 amine (NMEA)** S + 3 wt percentmethylethanol- 52.1 0.9 amine (NMEA) S + 3 wt percent methylethanol-50.3 1.1 amine (NMEA) DMTP* 5.8 −1.0 DMTP* 3.6 −1.1 DMTP + 3 wt percentdiethanol- 40.1 0.6 amine*not an Example of the present invention**spurious experimental outcomeS = SELEXOL ™ solvent

TABLE 2 Comparative Comparative Comparative Example 1 Example 1 Example2 Example 2 Example 3 Base Solvent SELEXOL ™ SELEXOL ™ SELEXOL ™ DMTPSELEXOL ™ Solvent Solvent solvent solvent Additive 0 Piperazine NMEAnone DBU Additive Concentration (wt percent) 25 3 3 0 3 # Absorber Trays20 25 25 25 25 # Stripper Trays 20 20 20 20 Total Gas Feed Rate (cc/min)2997 2996 2995 3026 3008 Feed CO2 (mole percent) 10.09 10.18 10.06 9.6411.06 Feed COS (mole percent) 1.48 1.51 1.51 1.21 1.63 Sweet CO2 (molepercent) 10.02 10.1 10.12 9.74 11.01 Sweet COS (mole percent) 1.47 1.071.41 1.14 1.59 Absorber: Lean H2O content (wt percent) 3.2 4 4.6 7.4 4.3Lean Solvent Flow (cc/min) 11 11 11 11 11 Lean T (F.) 80.7 78.8 79.879.8 80.5 Feed Gas T (F.) 80.8 79.1 79.5 79.3 80.3 Sweet Gas T (F.) 81.679.5 80.6 79.8 80.8 Rich T (F.) 79.3 77.6 78.1 77.9 77.1 Absorber P (inH2O) 28 28 28 28 28 Stripper Hot Rich Inlet T (F.) 239.1 220.3 232.5229.1 231.5 Overhead Vap T (C.) 103 100 103 100 100 Reboiler T (F.)264.7 258.3 254.6 270.5 259.5 COS Absorption (percent of feed) 0.7 29.16.6 5.8 2.5 CO₂ Absorption (percent of feed) 0.7 0.8 −0.6 −1.0 0.5

TABLE 3 Comparative Comparative Example 4 Example 5 Example 3 BaseSolvent SELEXOL ™ SELEXOL ™ SELEXOL ™ Solvent Solvent solvent AdditiveDBN DABCO DEA Additive 3 3 3 Concentration (wt percent) Number of 25 2525 Absorber Trays Number of 20 20 20 Stripper Trays Total Gas Feed 30002999 3004 Rate (cc/min) Feed CO2 (mole 10.9 10.26 10.3 percent) Feed COS(mole 1.6 1.51 1.48 percent) Sweet CO2 (mole 10.97 10.26 10.01 percent)Sweet COS (mole 1.55 1.47 1.2 percent) Absorber: Lean H2O content 4 3.64.1 (wt percent) Lean Solvent 11 11 11 Flow (cc/min) Lean T (F.) 80.782.3 81.1 Feed Gas T (F.) 80.3 81.8 81 Sweet Gas T (F.) 80.8 82.3 81.6Rich T (F.) 79.4 79.5 79.8 Absorber P (in 28 28 28 H2O) Stripper: HotRich Inlet 227.6 246.5 226.7 T (F.) Overhead Vap 100 102 102 T (C.)Reboiler T (F.) 259.9 260.6 256.1 COS Absorption 3.1 2.6 18.9 (percentof feed) CO₂ Absorption −0.6 0.0 2.8 (percent of feed)

TABLE 4 Comparative Comparative Example 4 Example 6 Example 7 Example 5Example 6 Base Solvent SELEXOL ™ SELEXOL ™ SELEXOL ™ SELEXOL ™ SELEXOL ™Solvent solvent solvent solvent solvent Additive MEA Quinuclidine noneDEA NMEA Additive Concentration (wt percent) 3 3 0 3 3 Number ofAbsorber Trays 25 25 25 25 25 Number of Stripper Trays 20 20 20 20 20Total Gas Feed Rate (cc/min) 2999 2978 2985 2990 3012 Feed CO2 (molepercent) 10.09 10.06 10.13 10.03 10.11 Feed COS (mole percent) 1.8 1.761.72 1.7 1.67 Sweet CO2 (mole percent) 10.16 10.08 10.11 10.02 10.02Sweet COS (mole percent) 1.29 1.67 1.7 1.43 0.8 Absorber Lean H2Ocontent (wt percent) 5.4 4.5 3.7 3.6 4 Lean Solvent Flow (cc/min) 11 1111 11 11 Lean T (F.) 79.7 80.7 82 83.2 80.9 Feed Gas T (F.) 80.6 79.980.5 81.8 83.2 Sweet Gas T (F.) 81.1 81 81.4 83 83.5 Rich T (F.) 78.676.7 77.8 80.9 82.5 Absorber P (in H2O) 28 28 28 28 28 Stripper: HotRich Inlet T (F.) 222.1 213.8 209.7 207.6 201.2 Overhead Vap T (C.) 109103 105 101 100 Reboiler T (F.) 246.6 250.2 257.5 256.9 255.1 COSAbsorption (percent of feed) 28.3 5.1 1.2 15.9 52.1 CO₂ Absorption(percent of feed) −0.7 −0.2 0.2 0.1 0.9

TABLE 5 Comparative Comparative Example 7 Example 8 Example 89 Example98 Example 9 Base Solvent SELEXOL ™ SELEXOL ™ DMTP SELEXOL ™ SELEXOL ™Solvent Solvent Solvent solvent Additive NMEA DIPA none HEP TEA AdditiveConcentration (wt percent) 3 3 0 3 3 Number of Absorber Trays 25 25 2525 25 Number of Stripper Trays 20 20 20 20 20 Total Gas Feed Rate(cc/min) 3023 3013 3004 2990 3000 Feed CO2 (mole percent) 10.08 10.0810.02 10.06 10.01 Feed COS (mole percent) 1.61 1.64 1.67 1.58 1.52 SweetCO2 (mole percent) 9.97 9.98 10.13 9.99 9.96 Sweet COS (mole percent)0.8 1.33 1.61 1.31 1.49 Absorber: Lean H2O content (wt percent) 3.7 48.2 3.2 3.6 Lean Solvent Flow (cc/min) 11 11 11 11 11 Lean T (F.) 79.482.9 82.4 84.2 81 Feed Gas T (F.) 78.4 81.3 81.1 82.4 79.3 Sweet Gas T(F.) 79.4 82.6 82 83.7 80.3 Rich T (F.) 78.3 80.1 79.6 80.2 77.5Absorber P (in H2O) 28 28 28 28 28 Stripper: Hot Rich Inlet T (F.) 200.2206.7 198.9 197.5 180.5 Overhead Vap T (C.) 101 100 104 100 101 ReboilerT (F.) 255.6 255.6 271.3 258.4 255.7 COS Absorption (percent of feed)50.3 18.9 3.6 17.1 2.0 CO₂ Absorption (percent of feed) 1.1 1.0 −1.1 0.70.5

TABLE 6 Compar- Compar- Compar- ative ative ative Exam- Exam- Exam-Exam- ple 10 ple 11 ple 10 ple 12 Base Solvent SELE- SELE- DMTP SELE-XOL ™ XOL ™ XOL ™ Solvent Solvent solvent Additive TEA MDEA DEA DMEAAdditive 3 3 3 3 Concentration (wt percent) Number of 25 25 25 25Absorber Trays Number of 20 20 20 20 Stripper Trays Total Gas 3005 29952981 2994 Feed Rate (cc/min) Feed CO2 10.17 10.01 10.09 9.95 (molepercent) Feed COS 1.48 1.52 1.52 1.47 (mole percent) Sweet CO2 9.9 9.8810.03 9.95 (mole percent) Sweet COS 1.49 1.5 0.91 1.44 (mole percent)Absorber: Lean H2O 4.2 3.8 8.6 4 content (wt percent) Lean Solvent 11 1111 11 Flow (cc/min) Lean T (F.) 82.6 82.6 80 80.3 Feed Gas T 80.4 80.479 79.7 (F.) Sweet Gas 81.9 81.8 79.6 81 T (F.) Rich T (F.) 77.8 78.579.4 76.9 Absorber P 28 28 28 28 (in H2O) Stripper: Hot Rich 185.2 189.5182.2 198.3 Inlet T (F.) Overhead 102 108 105 108 Vap T (C.) Reboiler T256.6 257.6 264.3 253.7 (F.) COS Absorption −0.7 1.3 40.1 2.0 (percentof feed) CO₂ Absorption 2.7 1.3 0.6 0.0 (percent of feed)

As can be seen from the Tables 1-6 hereinabove, certain amine additivesof the prior art improve removal of COS without substantially increasingthe removal of CO₂. For example, the addition of DABCO, quinuclidine,DBN and DBU to the SELEXOL™ solvent increases the COS removal of to theSELEXOL™ solvent from 1 percent to 2-5 percent.

EXAMPLES 11 TO EXAMPLE 15 AND COMPARATIVE EXAMPLES 13 TO 14

The performance of a physical solvent alone, and the mixture of thephysical solvent and prior art additives of the present invention wereevaluated in a bench scale glass absorber-stripper apparatus with thefollowing characteristics:

-   -   j) standard gas feed conditions (8.7-10 mole percent CO₂,        1.1-1.5 mole percent COS, and the balance nitrogen, about 3        L/min),    -   k) standard liquid feed conditions (about 11 or 30 cc/min),    -   l) standard amount of absorber and stripper staging (small        trays, 25 absorber, 20 stripper),    -   m) approximately standard amount of stripping heat duty applied,    -   n) standard amount of the additive used about 3 percent by        weight),    -   o) gas phase analysis by gas chromatography, liquid phase water        content by Karl Fisher method,    -   p) careful attention to ensuring steady state operation,    -   q) liquid phase COS content was not measured, and    -   r) stripper overhead not sampled for COS or H₂S.

The results obtained in the performance evaluation of different gastreating solvent are provided in Tables 7 and 8 below. TABLE 7 Exam-Comparative Exam- ple 12 Exam- Example 13 ple 11 003 ple 13 Base SolventSELEXOL ™ SELE- SELE- SELE- XOL ™ XOL^( ™) XOL ™ Additive None HEP HEPAEE Additive 0 3 3 3 Concentration (wt percent) # Absorber 25 25 25 25Trays # Stripper 20 20 20 20 Trays Total Gas 2985 2985 2985 2985 FeedRate (cc/min) Feed CO₂ 8.65 9.34 9.76 9.75 (mole percent) Feed COS 1.481.47 1.44 1.43 (mole percent) Sweet CO₂ 8.65 8.14 8.61 8.36 (molepercent) Sweet COS 1.33 0.52 0.58 0.41 (mole percent) Absorber Lean H₂O7.2 3.9 6.8 3.9 content (wt percent) Lean Solvent 30 30 30 30 Flow(cc/min) Lean T (F.) 81 85 84 85 Feed Gas T 89 89 89 89 (F)¹ Sweet Gas T66 72 72 70 (F.) Rich T (F.)² 83 88 88 87 Absorber P 28 28 28 28 (inH₂O) Stripper: Hot Rich 209 203 194 204 Inlet T (F.) Overhead 215 216215 214 Vapor T (F.) Reboiler T 115 122 118 132 (C.) COS Absorption 1065 60 71 (percent of feed) CO2 Absorption 0 13 12 14 (percent of feed)¹measured at outlet of feed gas heater²measured in pot beneath lowest absorber tray

As can be seen from the Table 7 hereinabove, the SELEXOL™ solvent aloneremoves about 10 percent of the COS from the feed gas while the additionof about 3 wt percent of HEP or AEE to the SELEXOL™ solvent improves COSremoval to 60-70 percent. Removal of CO₂ increases from 0 to 12-14percent. TABLE 8 Comparative Exam- Exam- Example 14 ple 14 ple 15 BaseSolvent DMTP DMTP DMTP Additive none HEP NMEA Additive Concentration (wt0 3 3 percent) # Absorber Trays 25 25 25 # Stripper Trays 20 20 20 TotalGas Feed Rate (cc/min) 2985 2985 2985 Feed CO₂ (mole percent) 9.75 9.739.66 Feed COS (mole percent) 1.43 1.43 1.42 Sweet CO₂ (mole percent)9.47 9.14 8.69 Sweet COS (mole percent) 1.31 0.45 0.0 Absorber: Lean H₂Ocontent (wt percent) 9.1 4.9 3.9 Lean Solvent Flow (cc/min) 11 11 11Lean T (F.) 76 79 82 Feed Gas T (F.)¹ 85 84 77 Sweet Gas T (F.) 71 72 68Rich T (F.)² 82 83 82 Absorber P (in H₂O) 28 28 28 Stripper: Hot RichInlet T (F.) 238 261 244 Overhead Vapor T (F.) 216 212 211 Reboiler T(C.) 129 149 150 COS Absorption (percent of feed) 8 69 100 CO2Absorption (percent of feed) 3 6 10¹measured at outlet of feed gas heater²measured in pot beneath lowest absorber tray

As can be seen from the Table 8 hereinabove, DMTP solvent alone removesabout 8 percent of the COS from the feed gas while the addition of about3 wt percent of HEP or NMEA to the DMTP solvent improves COS removal toabout 69-100 percent. Removal of CO₂ increases from 3 to 6-10 percent.

As can also be seen from the Tables 1-8 hereinabove, all the solventcompositions of the present invention showed a marked improvement in COSremoval with minimal additional CO₂ removal. Overall, the solventcompositions of the present invention comprising the SELEXOL™ solventand the alkanolamine of formula II or piparazine compound of formula IIIremoved COS in an amount of from 17 percent to 52 percent as compared tothe SELEXOL™ solvent alone which removed COS in an amount of only about1 percent. Similarly, the solvent composition of the present inventioncomprising DMTP and the alkanolamine of formula II removed COS in anamount of 40-100 percent as compared to DMTP alone which removed COS inan amount of from 3-8 percent. Such magnitude of improved removal of COSby the solvent compositions of the present invention is completelyunexpected and unpredictable.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

1. A solvent composition for selective removal of COS from a gas streamcontaining same, said composition comprising a) at least onepolyalkylene glycol alkyl ether of the formula R₁O-(Alk-O)_(n)—R₂  (I)wherein R₁ is an alkyl group having from 1 to 6 carbon atoms; R₂ ishydrogen or an alkyl group having from 1 to 4 carbon atoms; Alk is analkylene group, branched or unbranched, having from 2 to 4 carbon atoms,and n is from 1 to 10; and b) at least one alkanolamine compound of theformulaR₃NHR₄OR₆  (II) or at least one piperazine compound of formula

wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon atoms,or the R₄OH group; R₄ is a branched or unbranched alkylene group havingfrom 1 to 6 carbon atoms; R₅, independently in each occurrence, ishydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; andR₆ is hydrogen, an alkyl group having from 1 to 6 carbon atoms or anhydroxyalkyl group having from 1 to 4 carbon atoms.
 2. The solventcomposition according to claim 1 wherein the polyalkylene glycol alkylether of the formula I is a mixture of polyalkylene glycol alkyl etherscomprising dimethyl ethers of polyethylene glycols of formulaCH₃O(C₂H₄O)_(n)CH₃ wherein n is from 1 to
 10. 3. The solvent compositionaccording to claim 2 wherein the mixture of polyalkylene glycol alkylethers comprises from 0 to 0.5 wt percent of diethylene glycol dimethylether, from 5 to 7 wt percent of triethylene glycol dimethyl ether, from16 to 18 wt percent tetraethylene glycol dimethyl ether, from 23 to 25wt percent of pentethylene glycol dimethyl ether, from 22 to 24 wtpercent of hexaethylene glycol dimethyl ether, from 15 to 17 wt percentof heptaethylene glycol dimethyl ether, from 8 to 10 wt percent ofoctaethylene glycol dimethyl ether, from 3 to 5 wt percent ofnonaethylene glycol dimethyl ether, and from 1 to 2 wt percent ofdecaethylene glycol dimethyl ether.
 4. The solvent composition of claim1 wherein the component b) is an alkanolamine of formula II in whichsubstituent R₃ is hydrogen.
 5. The solvent composition of claim 1wherein the component b) is monoethanolamine.
 6. The solvent compositionof claim 1 wherein the component b) is an alkanolamine of formula II inwhich substituent R₃ is an alkyl group having from 1 to 6 carbon atomsor the R₄OH group.
 7. The solvent composition according to claim 6wherein the alkanolamine of formula II is selected from the groupconsisting of diethanolamine, methylethanolamine anddiisopropanoloamine.
 8. The solvent composition of claim 1 wherein thecomponent b) is piperazine.
 9. The solvent composition of claim 1wherein the component b) is hydroxyethylpiperazine.
 10. A process forselective removal of COS from a gas stream containing COS and CO₂, saidprocess comprising contacting the gas stream with a solvent compositioncomprising a) at least one polyalkylene glycol alkyl ether of theformulaR₁O-(Alk-O)_(n)—R₂  (I) wherein R₁ is an alkyl group having from 1 to 6carbon atoms; R₂ is hydrogen or an alkyl group having from 1 to 4 carbonatoms; Alk is an alkylene group, branched or unbranched, having from 2to 4 carbon atoms; and n is from 1 to 10; and b) at least onealkanolamine compound of the formulaR₃NHR₄OR₆  (II) or at least one piperazine compound of formula

wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon atoms,or the R₄OH group; R₄ is a branched or unbranched alkylene group havingfrom 1 to 6 carbon atoms; R₅, independently in each occurrence, ishydrogen or an alkyl group having from 1 to 6 carbon atoms or anhydroxyalkyl group having from 1 to 4 carbon atoms.
 11. The processaccording to claim 10 for selective removal of COS from a gas streamcomprising contacting the gas stream with the solvent composition asclaimed in claim
 2. 12. The process according to claim 10 for selectiveremoval of COS from a gas stream comprising contacting the gas streamwith the solvent composition as claimed in claim
 3. 13. The processaccording to claim 10 for selective removal of COS from a gas streamcomprising contacting the gas stream with the solvent composition asclaimed in claim
 4. 14. The process according to claim 10 for selectiveremoval of COS from a gas stream comprising contacting the gas streamwith the solvent composition as claimed in claim
 5. 15. The processaccording to claim 10 for selective removal of COS from a gas streamcomprising contacting the gas stream with the solvent composition asclaimed in claim
 6. 16. The process according to claim 10 for selectiveremoval of COS from a gas stream comprising contacting the gas streamwith the solvent composition as claimed in claim
 7. 17. The processaccording to claim 10 for selective removal of COS from a gas streamcomprising contacting the gas stream with the solvent composition asclaimed in claim
 8. 18. The process according to claim 10 for selectiveremoval of COS from a gas stream comprising contacting the gas streamwith the solvent composition as claimed in claim
 9. 19. A solventcomposition for selective removal of COS from a gas stream containingsame, said composition comprising a)1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; and b) at least onealkanolamine compound of the formulaR₃NHR₄OR₆  (II) or at least one piperazine compound of formula

R₅ wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbonatoms, or the R₄OH group; R₄ is a branched or unbranched alkylene grouphaving from 1 to 6 carbon atoms; R₅, independently in each occurrence,is hydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms;and R₆ is hydrogen, an alkyl group having from 1 to 6 carbon atoms or anhydroxyalkyl group having from 1 to 4 carbon atoms.
 20. A process forselective removal of COS from a gas stream containing COS and CO₂, saidprocess comprising contacting the gas stream with a solvent compositioncomprising a) 1,3-dimethyl-3,4,5,6-tetrahydro-2(1H)-pyrimidinone; and b)at least one alkanolamine compound of the formulaR₃NHR₄OR₆  (II) or at least one piperazine compound of formula

wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon atoms,or the R₄OH group; R₄ is a branched or unbranched alkylene group havingfrom 1 to 6 carbon atoms; R₅, independently in each occurrence, ishydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; andR₆ is hydrogen, an alkyl group having from 1 to 6 carbon atoms or anhydroxyalkyl group having from 1 to 4 carbon atoms.
 21. A solventcomposition for removal of COS from a gas stream containing same, saidcomposition comprising a) a mixture of N-formylmorpholine andN-acetylmorpholine; and b) at least one alkanolamine compound of theformulaR₃NHR₄OR₆  (II) or at least one piperazine compound of formula

wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon atoms,or the R₄OH group; R₄ is a branched or unbranched alkylene group havingfrom 1 to 6 carbon atoms; R₅, independently in each occurrence, ishydrogen or an hydroxyalkyl group having from 1 to 4 carbon atoms; andR₆ is hydrogen, an alkyl group having from 1 to 6 carbon atoms or anhydroxyalkyl group having from 1 to 4 carbon atoms.
 22. A process forselective removal of COS from a gas stream containing same, said processcomprising treating the gas stream with a solvent composition comprisinga) a mixture of N-formylmorpholine and N-acetylmorpholine; and b) atleast one alkanolamine compound of the formulaR₃NHR₄OR₆  (II) or at least one piperazine compound of formula

wherein R₃ is hydrogen, an alkyl group having from 1 to 6 carbon atoms,or the R₄OH group; R₄ is a branched or unbranched alkylene group havingfrom 1